Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an air or an oxygen-containing gas such as a gas chiefly containing oxygen is supplied to the cathode, and a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) or CO is supplied to the anode. The oxygen-containing gas and the fuel gas after used in the reaction are discharged as an exhaust gas for disposal.
The exhaust gas contains unconsumed fuel gas. It is not economical to dispose of all the unconsumed gas as the exhaust gas. In this regard, for example, as a technique of reducing the amount of disposal in the unconsumed fuel gas, a fuel cell as disclosed in Japanese Laid-Open Patent Publication No. 2002-151106 is known.
As shown in FIG. 19, the fuel cell is formed by stacking thin disks 1 each comprising solid electrolyte material or the like and ring plate separators 2 alternately, and providing ring plate path separators 3, 4 on both surfaces of the disks 1.
A cell reaction channel 5a is formed between one surface of the disk 1 and the separator 2 through the path separator 3, and an air channel 5b is formed between the other surface of the disk 1 and the separator 2 on the other side through the path separator 4.
A through hole 6 as a fuel gas channel extends through centers of the disks 1. A plurality of combustion gas channels 7 are formed around the through hole 6. The through hole 6 is connected to an inlet of the cell reaction channel 5a through a distribution inlet hole 8a, and an outlet of the cell reaction channel 5a is connected to each combustion gas channel 7 through a discharge hole 8b. 
The fuel gas (F) flows along the through hole 6, and flows into the cell reaction channel 5a through the distribution inlet holes 8a. The fuel gas turns back at the outer end of the disk 1, and flows into the combustion gas channel 7 through the discharge hole 8b. According to the disclosure, the unconsumed fuel gas discharged to the combustion gas channel 7 flows into the through hole 6 of a fuel cell (not shown) connected on the downstream side for allowing the unconsumed fuel gas to be used in the reaction again, and complete combustion is achieved.
In the conventional technique, the through hole 6 extends through centers of the disks 1, and a plurality of the combustion gas channels 7 are formed around the through hole 6. In the structure, the process of fabricating each disk 1 is considerably complicated, and special sealing structure is required. Further, since the shapes of the separator 2 and the path separators 3, 4 are complicated, the fuel cell does not have economical structure as a whole.
Further, the fuel gas before used in the reaction, and the fuel gas after used in the reaction are mixed together, and supplied to the cell reaction channel 5a sequentially. In the structure, concentration of the fuel gas supplied to each cell reaction channel 5a may not be uniform in the stacking direction. As a result, power generation reaction may differ depending on the disk 1.